To understand the possible role of the accumulation of mitochondrial DNA mutations in human aging and Alzheimer's disease (AD), it is important to identify the cellular populations that contain these abnormalities. We propose to test the hypothesis that the distinctive pattern of neuronal loss and morphological changes in AD reflects somatic mutations of mitochondrial DNA in cell populations vulnerable to disease and that this process is an acceleration of a ubiquitous phenomenon observed in normal aging. This Pilot will concentrate on the demonstraty by in situ hybridization (ISH) of the well-characterized 4977-base pair "common deletion" of DNA that has been demonstrated in Kearns-Sayre syndrome (KSS) and also in cardiac myocytes and human brain from aged individuals. In addition, we plan to modify the current strategy of mitochondrial DNA deletion ISH techniques with the development of a deletion-specific ISH method for studying formalin-fixed, paraffin-embedded brains. This technique will be developed through the study of tissue from a patient with a unique single deletion of mitochondrial DNA and KSS in which we have demonstrated a significant accumulation of mutant mitochondrial DNA forms in several organs. The deletion-specific ISH method will then be applied to material collected from patients with AD and age-matched controls, with an emphasis on localization within cells of the neocortex, basal ganglia, hippocampus, thalamus, substantia nigra, and locus coeruleus. These findings will then be correlated with known patterns of neuronal vulnerability in aging and in AD. It is expected that these studies will provide important new information concerning the contributions of mutated mitochondrial DNA and abnormal oxidative phosphorylation in the development of age-related changes in the human brain.